The Internet today has evolved into a stage at which many data communication networks (hereinafter referred to as “network”) are deployed around a system of fixed network node. These peripheral networks are known as edge networks, while the fixed network node system surrounded by the edge networks is known as a core network. With the emergence and development of wireless technologies, these edge networks are further used for wireless solutions. For example, as described in “Network Mobility Support Terminology” (Ernst, T., and Lach, H., Internet Draft: draft-ernst-nemo-terminology-01.txt, October 2002, Work in Progress) and “Network Mobility Support Requirements” (Ernst, T., and Lach, H., Internet Draft: draft-ernst-nemo-requirements-00.txt, October 2002, Work in Progress), a special edge network called a “mobile network”, that is, a moving network is being formed.
The mobile network is a network of a node at which the entire network changes its attachment points to the global network such as the Internet and usually requires a mobile router (device bridging the mobile network to the global network) within the mobile network between different access routers (actually, the access routers themselves may be mobile). For example, the mobile network includes a network attached to people (known as a personal area network, or PAN) and a network of sensors deployed in a vehicle such as car, train, ship or airplane. For a mass transport system such as airplane, train or bus, it is possible to provide passengers with onboard Internet access so as to connect a remote host using a laptop, personal digital assistance (PDA) or carphone. Individual nodes within such a mobile network are usually connected to a central apparatus (that is, mobile router), and when a network is in motion, the individual nodes do not change their attachment points and the mobile router changes the attachment points so that the entire network moves instead.
Therefore, the problem with the network in motion is different from the problem with conventional mobility support addressed by Mobile IPv4 (see “IP Mobility Support”, Perkins, C. E. et. al., IETF RCF 2002, October 1996) in IPv4 (see “Internet Protocol”, DARPA, IETF RFC 791, September 1981) and Mobile IPv6 (“Mobility Support in IPv6”, Internet Draft: draft-ietf-mobileip-ipv6-18.txt, Work in Progress, June 2002) in IPv6 (“Internet Protocol Version 6 (IPv6) Specification”, Deering, S., and Hinden, R., IETF RCF 2460, December 1998). A main objective of Mobile IPv4 and Mobile IPv6 is to provide mobility support to individual hosts rather than the entire network.
In a mobile IP, each mobile node has a unique home domain. When a mobile node is attached to its home network, a unique global address known as a home address is assigned to the mobile node. On the other hand, when a mobile node is away, that is, attached to some other foreign networks, a temporary global address known as a care-of-address is assigned to the mobile node. The idea of mobility support is to make it possible to reach the mobile node by referencing the home domain even when the mobile node is attached to other foreign networks. This is achieved by introducing an entity (home agent) to the home network.
The mobile node registers care-of-addresses with the home agent using a message known as “binding update.” The home agent is responsible for intercepting a message addressed to the home address of the mobile node. Also, the home agent is responsible for forwarding the packet to the care-of-address of the mobile node using IP-in-IP tunneling (see “IP-in-IP Tunneling”, Simpson, W., IETF RFC 1853, October 1995 and “Generic Packet Tunneling in IPv6”, Conta, A., and Deering, S., IETF RFC 2473, December 1998). The IP-in-IP tunneling involves encapsulating an original IP packet with another packet. The original packet may also be called “inner packet” and the new packet which encapsulates the inner packet may also be called “outer packet.”
When the concept of mobility support for individual hosts is extended to mobility support for a network of nodes, the objective of solving the problem of the mobile network is to make it possible to reach nodes within the mobile network using unique addresses no matter where on the Internet the mobile network is attached to. Several attempts have been made so far to provide a network mobility support. Most of them use a bidirectional tunnel between a mobile router and a home agent of the mobile router (see “Mobile Router Tunneling Protocol”, Kniveton, T., et. al., Internet Draft: draft-kniveton-mobrtr-03.txt, Work in Progress, November 2002, “Issues in Designing Mobile IPv6 Network Mobility with the MR-HA Bidirectional Tunnel (MRHA)”, Internet-Draft: draft-petrescu-nemo-mrha-00.txt, Work in Progress, October 2002, “IPv6 Reverse Routing Header and Its Application to Mobile Networks”, Thubert, P., and Molteni, M., Internet Draft: draft-thubert-nemo-reverse-routing-header-01.txt, Work in Progress, October 2002, and “Mobile Networks Support in Mobile IPv6 (Prefix Scope Binding Updates)”, Ernst, T., Castelluccia, C., Bellier, L., Lach, H., and Olivereau, A., Internet Draft: draft-ernst-mobileip-v6-network-03.txt, March 2002).
In a bidirectional tunnel between the mobile router and home agent, when in a home domain, the mobile router which controls the mobile network performs routing of packets on the mobile network using several routing protocols. On the other hand, when the mobile router and its network move to a foreign network, the mobile router registers the care-of-address with the home agent. Then, an IP-in-IP tunnel is set up between the mobile router and home agent. The mobile router uses the routing protocol used when it existed in the home domain on the IP-in-IP tunnel as well. This means that all packets directed to the mobile network are intercepted by the home agent and forwarded to the mobile router through the IP-in-IP tunnel. Then, the mobile router forwards the packet to a host in the mobile network. Furthermore, when the node in the mobile network wishes to send a packet to the outside of the network, the mobile router intercepts the packet, forwards the packet to the home agent through the IP-in-IP tunnel and then the home agent sends the packet to an intended recipient (packet destination set by the node within the mobile network).
However, the above described simple approach of the bidirectional tunnel cannot satisfy requirements of other powerful features (e.g., multi-homing) of IPv4 and IPv6 sufficiently. When there is a plurality of egress interfaces which offer independent routes to the global network, the mobile network can be multi-homed. When all these interfaces belong to the same router, only the router is multi-homed. The nodes of the mobile network which exist behind the router can only see one egress router and are not multi-homed. On the other hand, when these interfaces belong to different routers, the nodes of the mobile network see a plurality of egress routers and are therefore multi-homed.
The mobile network typically has wireless connection to the global network. In recent years, wireless technologies have made significant progress, but they could cause problems of instability of channels and noise, compared to wired networks. One of the advantages of multi-homing is the ability of a network node to use an alternative route to reach and be reached by the global network even when a certain uplink goes down.
However, according to the bidirectional tunnel mechanism used by the mobile router, nodes can only select one router as a default router. When this router loses its connection to the global network, the router cannot maintain the tunnel with the home agent. Moreover, a node using this router loses its connectivity to the global network even when another mobile router having an active link with the global network exists on the same network. Then, the nodes of the mobile network soon recognize that the default router has lost the route to the global network and selects an alternate mobile router as a default router.
Such a scheme depends on the nodes of the mobile network discovering routes for themselves, giving processing loads to the nodes having very limited processing capacity, e.g. embedded devices. Furthermore, there is a possibility that a delay may be produced for the nodes to recognize that the current default route has gone down. Moreover, different mobile routers broadcast prefixes of different subnets, and therefore when the mobile nodes eventually switch default routers, it is necessary to use different care-of-addresses and send a binding update to the home agent, which may further increase the delay in discovering routes.